1. Field of the Invention
This invention relates to an electrophotographic image-forming method called a "cleanerless system" in which the toner remaining on an electrophotographic photosensitive member after transfer is collected in at least the step of development so that any means exclusively used for cleaning can be omitted from assemblage; an electrophotographic image-forming appratus employing such a method; and a process cartridge used in this apparatus.
2. Related Background Art
Transfer type image-forming apparatuses (such as electrophotographic copying machines, fax machines and laser beam printers) are in wide use. They form images by forming on the surface of an electrophotographic photosensitive member (hereinafter often "photosensitive member") a toner image by an appropriate image-forming process and transferring the toner image to a transfer material such as paper, and fixing the transferred image to output an image-formed material (such as a copy or a print). The photosensitive member is repeatedly used for image formation.
In the transfer type image-forming apparatus, when the transfer of toner images from the photosensitive member side to the transfer material side is carried out, not all toner is transferred, but actually some part of the toner remains on the photosensitive member. The toner remaining on the photosensitive member is called "transfer residual toner." The transfer residual toner must be removed from the surface of the photosensitive member so that high-quality images free of stains, etc. can be obtained in repeated image-forming processes. Accordingly, it is necessary to remove the transfer residual toner from the surface of the photosensitive member (photosensitive member cleaning). It is common to provide an assembly exclusively used for cleaning (i.e., cleaner) such as a cleaning blade to remove the transfer residual toner from the surface of the photosensitive member.
In recent years, transfer type image-forming apparatus employing a cleanerless system have come into use in which the transfer residual toner remaining on the photosensitive member after transfer is collected in at least the development step and is reused (i.e., cleaning-at-development).
The image-forming apparatuses employing such a cleanerless system are effective in view of ecology and also enable image-forming apparatuses to be made small-sized, light-weight and low-cost.
Meanwhile, as a charging means for uniformly charging the photosensitive member to stated polarity and potential, corona charging assemblies have commonly been used. This is a means in which a corona charging assembly is provided in non-contact and face to face relationship with a photosensitive member, and the surface of the photosensitive member is exposed to a corona shower generated from the corona charging assembly to which high voltage is applied.
In recent years, rather than such charging assemblies, contact charging assemblies are brought into practical use because of their advantages of, e.g., low ozone and low power consumption.
In the contact charging assemblies, a conductive charging member (contact charging member) of a blade type or a fur brush type is brought into contact with a charging target such as the photosensitive member, and a stated charging voltage (charging bias) is applied to this contact charging member to charge the charging target to stated polarity and potential.
In the charging structure (the mechanism of charging), two types, a corona charging type and an injection charging type, stand intermingled. Which performance comes out depends on which is predominant.
In an image-forming apparatus employing a cleanerless system which makes use of such a contact charging assembly as a charging means for the photosensitive member, the transfer residual toner on the photosensitive member is carried to an area where the contact charging member and the photosensitive member come into contact (i.e., a charging zone) and adheres to and mingles with the contact charging member, thus it is temporarily collected by the contact charging member (i.e., cleaning-at-charging).
The toner collected by the contact charging member is, after its charge polarity is adjusted, successively electrically sent out from the contact charging member onto the photosensitive member.
The toner sent out from the contact charging member onto the photosensitive member is carried to a developing zone, which is an area where a developing assembly as a developing means faces the photosensitive member, and is collected at the developing assembly (cleaning-at-development) and reused.
In the cleaning-at-development, the transfer residual toner on the photosensitive member can be collected by the aid of a fog-removing bias (a defogging potential difference Vback which is a potential difference intermediate between the DC component applied to a developing member of the developing assembly and the surface potential of the photosensitive member) at the time of the next and subsequent development on the photosensitive member.
Incidentally, the toner sent out from the contact charging member onto the photosensitive member is usually in a small quantity and in a state of a very thin layer standing scattered uniformly, and does not substantially adversely affect the next step of imagewise exposure. Also, ghost images caused by transfer residual toner patterns are also prevented from occurring.
The transfer residual toner on the photosensitive member often comes to have a charge polarity reversed as a result of, e.g., discharging at the time of transfer. For the toner whose polarity is reversed, it is difficult to collect at the developing assembly simultaneously with development (i.e., by cleaning-at-development). The contact charging member takes in toner containing such toner whose polarity has turned reverse, adjusts it to a regularly charged toner and then sends out the toner onto the photosensitive member. Hence, the transfer residual toner can be collected easily at the developing assembly simultaneously with development.
In order to satisfy various performances, metal oxides commonly called external additives are added to toners. For example, as disclosed in Japanese Patent Application Laid-Open No. 61-275862 and No. 61-275863, it is proposed that alumina having been made hydrophobic is used to stabilize triboelectric chargeability of toners.
Japanese Patent Application Laid-Open No. 48-47345 proposes a metal oxide powder as an abrasive; and Japanese Patent Application Laid-Open No. 52-19535 and No. 56-128956, a metal oxide such as titanium oxide as a flowability-providing agent. Also, Japanese Patent Application Laid-Open No. 4-337739, No. 4-348354, No. 4-40467 and No. 5-72797 discloses use of a surface-treated amorphous titanium oxide powder for the purposes of imparting flowability, stabilizing the chargeability of toners and preventing toners from causing filming.
These are all inorganic matters, and may rub the surface of the photosensitive member to cause wear of the photosensitive member surface.
When such a toner with an external additive is used in the above image-forming apparatus of cleanerless system making use of the contact charging assembly as a charging means for the photosensitive member, the transfer residual toner on the photosensitive member is carried to the contact charging member/photosensitive member contact area (the charging zone) and adheres to and mingles with the contact charging member. Hence, there is such a problem that the external additive tends to rub and wear the photosensitive member at the contact charging member/photosensitive member contact area, consequently tending to especially cause faulty images.